FIG. 1 is a schematic diagram depicting a typical prior art optical receiver 10. Optical signal 109 is converted to electrical signal 110 at PIN detector 101 which is biased through bias circuit 108, and is driven into transimpedance amplifier (TIA) 102. In prior art receivers, output signal 111 from transimpedance amplifier 102 may be coupled through DC blocking capacitor 103 into input signal 112 of limiting amplifier 104, where it is compared with decision threshold level input signal 106, resulting in a limited output signal 105. Limiting amplifier 104, sometimes referred to as a decision circuit, is a high speed, high sensitivity comparator that quantizes the analog signal coming from TIA 102. The purpose of limiting amplifier 104 is to convert noisy, linear, analog output signal 111 from TIA 102 into well-defined high and low logic levels. The ability of limiting amplifier 104 to accurately determine if input signal 112 is logic high or logic low is highly dependent on the level and stability of decision threshold level signal 106. For a given input optical amplitude level, there is an optimal decision threshold level, which is usually at the midpoint between high and low logic levels from TIA 102. Small variations in the decision threshold level can directly affect the output crossing level, possibly resulting in logic level errors. Therefore, the quality and error rate of a receiver is directly dependent on how accurately the decision threshold level is set for a given input optical power level.
In conventional (non-burst-mode) optical transmission systems, the average power level of input optical signal 109 remains relatively constant. This facilitates the use of capacitive coupling through DC blocking capacitor 103 between transimpedance amplifier 102 and limiting amplifier (LA) 104, allowing the use of a fixed decision threshold level 106 in limiting amplifier 104. The average power level of optical signal 109 is relatively constant, varying due to thermal and aging effects, which typically occur slowly over long periods of time. However, there may still be a need to actively control the decision threshold level, or the gain of the TIA, in order to compensate for various offsets in the circuit and the slowly moving thermal and aging effects. There are examples in the prior art of circuits and systems intended to compensate for such variations. One example may be found in U.S. Pat. No. 6,191,879 (issued Feb. 20, 2001) in which the current of a PIN photo diode is controlled in order to cancel offsets in the system. Another example is found in U.S. Pat. No. 5,307,196 (issued Apr. 26, 1994) in which an offset voltage is added directly to the compare reference value.